The declining population of monk seals on the Ionian islands of Kefalonia, Ithaca, and Lefkada, Greece, was studied from July 1986 to April 1988. The study included (1) individual identification, (2) number of sightings, (3) use of caves, (4) damage to fishing gear, and (5) deaths. Three hundred and ninety-seven sightings of about 18 seals (including eight pups) were recorded. Maximum sightings occurred in June/July 1987; most sightings were of solitary animals. Twenty of 126 surveyed caves (16%) were used by seals. There were preferences for specific caves. There was no evidence of a diurnal pattern of cave use. Fishing trips near the study sites were monitored, and 136 of 1864 (7·3%) reported damage by seals to fishing gear. Significant correlations were found between sightings, cave use, and damage to fishing gear and/or catch. In an experiment we demonstrate that one seal may cause considerable damage in one night. Mortality data of 25 years show that most of 34 reported deaths were caused by deliberate killing (62%) and accidents in fishing gear (24%). Suggested measures for mitigating the decline of monk seals include (1) establishing protection zones, (2) compensating fishermen for losses, and (3) expanding public awareness programmes.